Transparent armor is a material that is designed to provide ballistic protection while remaining optically transparent. This type of material can be used in combat and non-combat situations (e.g. riot control) for windows (both vehicles and buildings), protective visors, and protective covers for equipment such as sensing devices among other uses. While there may be specific requirements for each particular use, there are requirements that are common to most systems or devices. Specifically, the primary requirement for a transparent armor is that it not only defeat a specific threat, but that it also have the ability to receive multiple hits without distorting vision in the area surrounding the hit. Additional requirements, which may have to be optimized, depend on the particular use. These addition requirements include weight, space efficiency and cost-versus-performance. While many problems with transparent armor can be cured by increasing the thickness of the armor, this solution is not desirable because this increases the weight that has to be carried by a person or vehicle, increases distortion and thus reduces transparency due to the thickness of the material, and, in vehicles, is impractical due to space limitations.
Transparent materials that are used for ballistic protection (transparent armor) include:                (a) Polymeric materials, the most common being polycarbonate. This is an inexpensive material that can easily be fabricated and offers very good protection against small fragments. It is generally used for goggles, visors, face shields and eye “glasses”. Other plastics such as transparent nylons, acrylates and polyurethanes have also been investigated, but their durability (e.g. to ultraviolet radiation) and optical properties limit their applications.        (b) Conventional glasses, such as soda lime and borosilicate glass, typically manufactured using the float process. These materials are low-cost, but increased requirements for lower weight, improved optical properties and ballistic performance have generated the need for improved materials.        (c) Crystalline materials such as aluminum oxynitride (AlON), single crystal aluminum oxide (Sapphire) and spinel (MgAl2O4) are the major materials presently being considered.        (d) Glass-ceramic Materials                    (i) One glass-ceramic material is TransArm™, a lithium disilicate glass-ceramic from Alstom UK Ltd. Due to its superior weight efficiency against ball rounds and small fragments, TransArm has the potential to increase performance of protective devices such as face shields used for explosive ordnance disposal. Studies of the shock behavior of these materials have shown that the glass-ceramic has a high post-failure strength compared to that of amorphous glasses.            (ii) U.S. Pat. No. 5,060,553 (Jones, 1991) describes armor material based on glass-ceramic bonded to an energy-absorbing, fiber-containing backing layer. Glass compositions listed in the patent that could be used to produce glass-ceramic materials include lithium zinc silicates, lithium aluminosilicates, lithium zinc aluminosilicates, lithium magnesium silicates, lithium magnesium aluminosilicates, magnesium aluminosilicates, calcium magnesium aluminosilicates, magnesium zinc silicates, calcium magnesium zinc silicates, zinc aluminosilicate systems calcium phosphates, calcium silicophosphates and barium silicate. While the transparency of the resulting glass-ceramic compositions was not specified, the use of a fiber-filled backing layer is likely to render these composites opaque.            (iii) U.S. Pat. No. 5,496,640 (Bolton and Smith, 1996) describes fire- and impact-resistant transparent laminates comprising parallel sheets of glass-ceramic and polymer, with intended use for security or armor glass capable of withstanding high heat and direct flames. Materials listed in the patent include commercial plate glass, float or sheet glass compositions, annealed glass, tempered glass, chemically strengthened glass, PYREX® glass, borosilicate glasses, lithium containing glasses, PYROCERAM, lithium containing ceramics, nucleated ceramics and a variety of polymer materials.                        
In addition to the materials mentioned above, additional materials and methods have also been investigated for ballistic protection. U.S. Pat. No. 5,045,371 (Calkins, 1991) describes a glass composite armor having a soda-lime glass matrix with particles of a pre-formed ceramic material dispersed throughout the material. The ceramic material was thus not grown in situ as is the case with glass-ceramics. U.S. Patent Application No. 2005/0119104 A1 (Alexander et al) describes an opaque, not transparent, armor based on anorthite [CaAl2Si2O8] glass-ceramics.
While the materials and method described above have afforded ballistic protection, improvements in the area of transparent armor material systems are sorely needed. As the AMPTIAC Newsletter, Fall 2000, has stated: “Future warfighter environments will require lightweight, threat adjustable, multifunctional and affordable armor, which the current glass/polycarbonate technologies are not expected to met.” The present invention is specifically directed to new low-cost, threat-effective material system transparent armor systems that will met these requirements.